In an optical fiber radio system, enhancement of input intensity to an optical fiber leads to an increase of an RF link gain, extension of a transmission distance, and the like, and therefore is an important issue. However, when the light intensity input to the optical fiber is strengthened, it becomes impossible to perform linear transmission of the signal because it is affected by the non-linear effect caused in the optical fiber. Thus, the light intensity of optical input is limited to several mW in the current optical fiber radio systems.
Meanwhile, complicated optical multiplexing systems such as a coherent optical transmission system of optical phase shift keying (optical PSK), optical quadrature amplitude modulation (optical QAM), and the like, and optical orthogonal frequency division multiplexing (optical OFDM) have also attracted attention in optical fiber transmission, optical fiber radio transmission and the like. In the case of transmitting these optical signals, influence of an optical non-linear effect in a transmission path cannot be ignored, and there is a strong demand for a compensation method thereof.
JP 2010-28470 A discloses an optical signal receptor which includes an electric operating circuit for compensation of a non-linear waveform distortion and the compensation of the non-linear distortion using an optical phase conjugate pair signal. However, when the waveform distortion caused by the non-linear effect in an optical transmission path is compensated using the electric circuit, there is a problem that the device becomes complicated, a problem that providings of a plurality of optical modulators using a pre-processed electric signal obtained by digital signal processing or the like for the generation of the optical phase conjugate pair signal becomes necessary on the transmitter side, and a problem that performance of electrical signal processing using digital signal processing or the like becomes necessary on the receiver side.